Selection of Generator for SHP (Draft), AHEC Roorkee, India

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DRAFT

May 29, 2008

STANDARDS / MANUALS / GUIDELINES FOR

SMALL HYDRO DEVELOPMENT

SPONSOR:

MINISTRY OF NEW AND RENEWABLE ENERGY

GOVERNMENT OF INDIA

GUIDELINES FOR SELECTION OF

HYDRO GENERATOR

FOR SHP

LEAD ORGANIZATION:

ALTERNATE HYDRO ENERGY CENTRE

INDIAN INSTITUTE OF TECHNOLOGY, ROORKEE

AHEC/MNRE/SHPStandards/GuidelinesForSelectionofHydroGeneratorForSHP

CONTENTS

Page No.

1. GENERAL 1

1.1 References and Codes 1

2. HYDRO GENERATOR ABOVE 5 MVA 2

2.1 General 2

2.2 Site Operating Conditions (as per IEC: 60034 & IEEE C-50-12) 2

2.3 Transient event and emergency duty requirements 4

2.4 Rotor Surface Heating 6

2.5 Types of Generators and Configuration (Vertical or Horizontal) 7

2.6 Capacity and Rating 7

3. ELECTRICAL CHARACTERISTICS 8

3.1 Generator Terminal Voltage 8

3.2 Insulation and Temperature Rise 8

3.3 Short Circuit Ratio 9

3.4 Line Charging and Synchronous Condensing Capacity 9

3.5 Reactance 10

3.6 Damper Winding 10

3.7 Efficiency 11

3.8 Total Harmonic Distortion (THD) 11

4. MECHANICAL CHARACTERISTICS 11

4.1 Direction of Rotation 11

4.2 Rotor Assembly Critical Speeds 11

4.3 Phase Sequence 12

4.4 Noise Level and Vibration 12

4.5 Over speed withstand 12

4.6 Flywheel Effect 13

4.7 Cooling 13

4.8 Fire Extinguishing System 14

5. SMALL HYDRO GENERATOR UPTO & BELOW 5 MVA 14

5.1 General 14

5.2 Classification Of Generators 14

5.3 Selection and Characteristics 17

5.4 Vertical/Horizontal Configuration 18

5.5 Speed (rpm) 18

5.6 Dimension 18

5.7 Overspeed Withstand 18

5.8 Ratings and Electrical Characteristics 18

5.8.1 kW Rating 18

AHEC/MNRE/SHPStandards/GuidelinesForSelectionofHydroGeneratorForSHP

5.8.2 kVA Rating and power factor 19

5.8.3 Frequency and Number of Phases 19

5.8.4 Generator Terminal Voltage 19

5.8.5 Stator Winding Connection 19

5.8.6 Excitation Voltage 19

5.9 Synchronous Generators 20

5.10 Asynchronous (Induction) Generator 20

5.11 Guide and Thrust Bearings 21

5.12 Generator Efficiencies 21

5.13 Testing of Generator 21

5.13.1 Factory Assembly Test 22

5.13.2 Field Acceptance Test 22

6. EXCITATION SYSTEM 23

6.1 General 23

6.2 Excitation System Type 23

6.3 Steady State Excitation System Requirement 25

6.3.1 Rated Field Current 25

6.3.2 Exciter Rated Current 25

6.3.3 Exciter rated Voltage 25

6.3.4 Rated Field Voltage 25

6.4 Transient Requirements 25

6.4.1 Transient requirements 25

6.4.2 Ceiling Voltage 26

6.4.3 Excitation System Nominal Response 27

6.5 Power System Stabilizer 27

6.6 Under Excitation Limiter 28

6.7 Over excitation limiter 28

6.8 Volts-per Hertz (V/Hz) Limiter 28

6.9 VAR or PF Control System 28

6.10 Redundancy of Equipment 28

6.11 Environmental Considerations 28

6.12 Equipment Tests 29

6.12.1 Static Excitation (potential source rectifier exciter) system 29

6.12.2 Rectifier Assembly 29

6.12.3 Brushless Excitation System 30

7. EXAMPLE 30

Annexure-1 31

Annexure-2 36

Annexure-3 37

Annexure-4 40

Annexure-5 41

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Guide for Selection of Hydro-Generators

and Excitation System Upto 25MW

1 GENERAL

The electric generator converts the mechanical energy of the turbine into electrical

energy. The two major components of the generator are the rotor and the stator. The rotor

is the rotating assembly to which the mechanical torque of the turbine shaft is applied.

By magnetizing or “exciting” the rotor, a voltage is induced in the stationary component,

the stator. The principal control mechanism of the generator is the exciter-regulator

which sets and stabilizes the output voltage. The speed of the generator is determined by

the turbine selection, except when geared with a speed increaser. In general, for a fixed

value of power, a decrease in speed will increase the physical size and cost of the

generator.

The location and orientation of the generator is influenced by factors such as turbine type

and turbine orientation. For example, the generator for a bulb type turbine is located

within the bulb itself. A horizontal generator is usually required for a tube turbine and a

vertical shaft generator with a thrust bearing is appropriate for vertical turbine

installations.

Conventional cooling on a generator is accomplished by passing air through the stator

and rotor coils. Fan blades on the rotating rotor assist in the air flow. For larger generator

(generally above 5 MW capacity) and depending on the temperature rise limitations of

the winding insulation of the machine, the cooling may be assisted by passing air through

surface air coolers, which have circulated water as the cooling medium.

The Generators interconnected with the grid should need grid standards issued by CEA

Relevant extracts – Schedule for operation and maintenance are enclosed as annexure-1)

1.1 References and Codes

Latest edition of the following standards are applicable.

IEC-1116: 1992 – Electro-Mechanical Equipment Guide for Small Hydro-electric

Installation

IEC-34-1: 1983 – Rotating Electrical Machines, Rating and Performance

IEC-34-2A-1972 - Rotating Electrical Machines

Methods for determining losses and efficiency of electrical

machinery from tests (excluding machines for traction vehicles

IEC-34-5-1991 – Classification of degrees of protection provided by enclosures for

rotating electrical machines (IP Code)

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IEC-85-1987 - Classification of materials for the insulation of electrical machines

IS-4722 –1992 Rotating electrical machines

IS-325 –1996 Three phase induction motor

IS-8789 –1996 Values of performance characteristics for three phase induction

motors

ANSI/IEEE 1010-197-American National Standard IEEE Guide for Control of Hydro

Power Plants

Micro hydel Std. AHEC IIT Roorkee

2 HYDRO GENERATOR ABOVE 5 MVA

2.1 General

Hydraulic turbine driven generators for hydro plants are salient pole synchronous

alternating current machines. Large salient pole generators are relatively slow speed

machines in the range 80-375 rpm with large number of rotor poles. These generators are

normally specifically designed and generally interconnected with grid.

2.2 Site Operating Conditions (as per IEC: 60034 & IEEE C-50-12)

Rated operation condition specified in the standards are as follows: Site operating

conditions if deviating from these value, correction have to be applied.

Maximum Ambient Temperature Steady State duty: Salient-pole open ventilated air-

cooled synchronous generators operate successfully when and where the temperature of

the cooling air does not exceed 40

Salient-pole totally enclosed water to air cooled (water) synchronous generators operate

successfully when and where the secondary coolant temperature at the inlet to the

machine or heat exchanger do not exceed 25

If the cooling air temperature (ambient) exceeds 40

C, or cooling water temperature

exceeds 25

C then maximum allowable temperature based on temperature rise on

reference temperature (40

/25

C) of the insulation class be specified instead of

temperature rise.

The minimum temperature of the air at the operating site is – 15

C, the machine being

installed and in operation or at rest and de-energized.

Note: If temperatures different from above are expected. The manufacturer should be

informed of actual site conditions.

Generators: Generators should operate successfully at rated MVA, frequency, power

factor, and terminal voltage. Generators at other service conditions should be specified

with the standards of performance established at rated conditions.

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Altitude: Height above sea level not exceeding 1000 m. For machines intended for

operation on a site where the altitude is in excess of 1000 m. should be specifically

brought out.

Number of starts and application of load: The purchaser should specify the anticipated

no. of starts and maximum MVA, power, and reactive power loading rate of change

requirements for the manufacturer to take into account in the machine design. The

method of starting must be identified in the case of peaking stations.

Variation from rated voltage and frequency: Generators should be thermally capable

of continuous operation within the capability of their reactive capability curves over the

ranges of ± 5 % in voltage and ± 2 % in frequency, as defined by the shaded area of

figure 2.2.

a) As the operating point moves away from rated values of voltage and frequency, the

temperature rise of total temperatures of components may progressively increase.

Continuous operation near certain parts of the boundary of the shaded area in figure

2.2 (a) at outputs near the limits of the generator’s reactive capability curve may

(figure 2.2 b) cause insulation to age thermally at approximately two times to six

times its normal rate.

b) Generators will also be capable of operation within the confines of their reactive

capability curves within the ranges of ± 3 %/ -5 % in frequency as defined by the

outer boundary (zone B) in figure 2.2 (a) with further reduction of insulation life.

c) To minimize the reduction of the generator’s lifetime due to the effect of temperature

and temperature differentials, operation outside the shaded area should be limited in

extent, duration, and frequency of occurrence. The output should be reduced or other

corrective measures taken as soon as practicable.

d) The boundaries of figure 2.2 (a) result in the magnetic circuits of the generator to be

over fluxed under fluxed by no more than 5%. The sloped boundaries in figure (2.2

(a) correspond to constant voltz per hertz.

e) The machine may be unstable or margins of stability may be reduced under some of

the operating conditions shown in fig. 2.2 (a). Excitation margins may also be

reduced under some of the operating conditions shown in figure 2.2 (a).

f) As the operating frequency moves away from the rated frequency, effects outside the

generator may become important and need to be considered. For example, the turbine

manufacturer will specify ranges of frequency and corresponding periods during

which the turbine can operate, and the ability of the auxiliary equipment to operate

over a range of voltage and frequency should be considered.

g) Operation over a still wider range of voltage and frequency, if required, should be

subject to agreement between the purchaser and the manufacturer and need to be

specifically brought out in tender specification.

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Key 1 zone A

X axis frequency p. u. 2 zone B (outside zone A)

Y axis voltage p. u. 3 rating point

Fig. 2.2 (a) Voltage and Frequency Limits for Generators

(As per IEC: 60034)

2.3 Transient event and emergency duty requirements

A generator confirming to these guidelines will be suitable for withstanding exposure to

transient event and emergency duty imposed on a generator because of power system

faults.

Sudden short circuit at the generator terminals: A generator will be capable of

withstanding, without injury, a 30 second, 3 phase short circuit at its terminals when

operating at rated MVA and power factor and at 5% over voltage, with fixed excitation.

The machine shall also be capable of withstanding, without injury, any other short circuit

at its terminals of 30 s duration or less in accordance with IEEE C 50. 12-2005.

Generator circuit breaker need to be selected accordingly.

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SYSTEM STABILITY

LIMIT

LINE CHARGING LIMIT

MINIMUM

EXCITATION

LIMIT

UNDEREXCITED

(LEADING)

MEGAVARS OVEREXCITED

(LAGGING)

CAVITATION

LIMIT

LIMITED BY FIELD HEATING

POWER FACTOR

RATED MVA LIMITED BY

STATOR HEATING

MEGAWATTS

SHAFT STRESS OR

HYDRAULIC LIMIT

SEE NOTE-1

SEE NOTE-2

Fig. 2.2 (b) Typical Hydro-Generator capability Curve

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Synchronizing

a. Generators are designed to be fit for service without inspection or repair after

synchronizing that is within the limits given below:

i) Breaker closing angle ±10%

ii) Generator side voltage relative to system 0% to +5%

iii) Frequency difference ±0.067 Hz

Additional information on synchronizing practices can be found in IEEE std. C37. 102

1995.

b. Faulty synchronizing is that which is outside the limits given above. Under some

system conditions, faulty synchronizing can cause intense, short duration currents

and torques that exceed those experienced during sudden short circuits. These

currents and torques may cause damage to the generator.

c. Generators shall be designed so that they are capable of coasting down from

synchronous speed to a stop after being immediately tripped off-line following a

faulty synchronization. Any generator that has been subject to a faulty

synchronization shall be inspected for damage and repaired as necessary before

being judged fit for service after the incident. Any loosening for stator winding

bracing and blocking and any deformation of coupling bolts, couplings, and rotor

shafts should be corrected before returning the generator to service. Even if

repairs are made after a severe out-of-phase synchronization, it should also be

expected that repetition of less severe faulty synchronizations might lead to

further deterioration of the components.

d. It should be that the most severe faulty synchronizations, such as 180

or 120

out-of-phase synchronizing to a system with low system reactance to the infinite

bus, might require partial or total rewind of the stator, or extensive or replacement

of the rotor, or both.

Check synchronizing relay and auto synchronizing equipment need to be set

accordingly. Normally synchronizing closing angle is kept ±7%.

Short-time volts/hertz variations: The manufacturer shall provide a curve of safe short-

time volts/hertz capability. Identify the level of overflux above which the machine should

never be operated, to avoid possible machine failure. Unless otherwise specified, the

curve apply for time intervals of less than 10 min.

2.4 Rotor Surface Heating

Continuous phase current unbalance: Generator above 5 MVA are normally capable of

withstanding, without injury, the effects of a continuous phase current unbalance

AHEC/MNRE/SHPStandards/GuidelinesforselectionofHydroGeneratorforSHPPage7

corresponding to a negative current of the values in table 1, providing the rated MVA is

not exceeded and the maximum as expressed as a percentage of rated stator current.

Table –1 Continuous negative sequence current capability

Type of generator or

generator/motor

Permissible I

(%)

Non-connected amortisseur winding 5

Connected amortisseur winding 10

These values also express the negative-sequence current capability at reduced generator

MVA capabilities, as a percentage of the stator current corresponding to the reduced

capability.

Continuous performance with nonconnected amortisseur windings is not readily

predictable. Therefore, if unbalanced conditions are anticipated, machines with connected

amortisseur windings should be specified. Negative sequence relays (phase unbalance) be

set accordingly.

2.5 Types of Generators and Configuration (Vertical or Horizontal)

Vertical shaft generators are generally used. There are two types of vertical shaft hydro

generators distinguished by bearing arrangements.

Umbrella type generators: These generators have combined bottom thrust and guide

bearings and confined to low operating speeds (upto 200 rpm) and is the least expensive

generator design. In semi umbrella type generators a top guide bearing is added.

Umbrella/Semi Umbrella design is being increasingly used for slow speed vertical

generator.

Conventional generators: Prior to introduction of umbrella and semi umbrella designs

conventional design comprised of top-mounted thrust and guide bearing supported on

heavy brackets, capable of supporting total weight of generator. A bottom guide bearing

combined with turbine shaft is usually provided. This conventional design is used for

high speeds (upto 1000 rpm) generators. Some medium size low flow turbine and tube

turbine generators are horizontal shaft. Direct driven bulb turbine generators are also

horizontal shaft generators located in the bulb. Pelton turbine coupled generators are

mostly horizontal shaft.

2.6 Capacity and Rating

kW Rating: kW capacity is fixed by turbine rated output. In a variable head power plant

the turbine output may vary depending upon available head. In general the generator is

rated for turbine output at rated head. In peaking power plant higher generator kW rating

could be specified to take care of possible higher turbine output. Economic analysis is